Refractory fibers of zirconia and silica mixtures

ABSTRACT

Refractory fibers of zirconia and silica mixtures are made by shaping and dehydratively gelling, for example by extruding in air, an aqueous mixture of a zirconium compound, such as zirconium diacetate, and colloidal silica, and heating the resulting gelled fiber in a controlled manner to decompose and volatilize undesired constituents and convert the fiber to a refractory fiber having a desired microstructure and useful to form refractory fabrics, reinforced composites, heat or sound insulation, filter or adsorption media, etc.

United States Patent [1 1 Sowman [4 1 Feb. 19, 1974 [75] Inventor:Harold G. Sowman, Maplewood,

Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul,Minn.

[22] Filed: Aug. 2, 1971 [21] Appl. No.: 168,298

Related US. Application Data [62] Division of Ser. No. 825,273, May [6,1969, Pat. No.

UNITED STATES PATENTS 3,082,099 3/l963 Beasley et al 106/57 3/1967Sterry et al. 106/57 Primary Examiner-James E. Poer Attorney, Agent, orFirm-Alexander, Sell, Steldt & Delahunt 5 7] ABSTRACT Refractory fibersof zirconia and silica mixtures are made by shaping and dehydrativelygelling, for example by extruding in air, an aqueous mixture of azirconium compound, such as zirconium diacetate, and colloidal silica,and heating the resulting gelled fiber in a controlled manner todecompose and volatilize undesired constituents and convert the fiber toa refractory fiber having a desired microstructure and useful to formrefractory fabrics, reinforced composites, heat or sound insulation,filter or adsorption media, etc.

21 Claims, 4 Drawing Figures 'Pmmnrww V 3,793,041

INVENTOR. HAROLD G Sow/WAN BY MJWJMM ATTORNEYS REFRACTORY FIBERS OFZIRCONIA AND SILICA MIXTURES This application is a division of copendingapplication, Ser. No. 825,273, filed May 16, 1969, now U.S. Pat. No.3,709,706.

This invention relates to refractory material or articles of zirconiaand silica mixtures, such as fibers as well as to articles madetherefrom, such as textiles, or articles resulting from the associationof such refractory material with other materials, such as compositescontaining such refractory material as reinforcement. In another aspect,it relates to transparent, strong, flexible, continuous, round, smooth,and glossy fibers of a mixture of zirconia and silica, which can beinternally colored, which fibers can be used as composite reinforcementor to make refractory articles such as fabrics. In another aspect, itrelates to a process for the preparation of such refractory fibers.

Synthetic refractory metal oxide articles have been made from melts fora long time and more recently by vapor deposition. The latter processhas been used to a limited extent to make small quantities of relativelyshort alumina and beryllia fibers (as well as boron fibers), but withthe present state of the art, such products are expensive and difficultto obtain. Silica fibers are also now obtainable, though they arerelatively expensive and lose strength at high temperatures, such fibersbeing made by melt processes or by leaching and heat treatingborosilicate glass. Refractory articles made from melts, such as sheetglass and glass fiber, have enjoyed wide-spread use, but theirapplication has been limited by their fragility and brittleness andtheir relatively low melting or softening points. The relativeproportions of inorganic oxide materials which can be used in such meltprocesses to make useful articles are limited, and the melt processesthemselves have the disadvantage of requiring the use of hightemperatures to melt the components and costly equipment to handle themelts. For example, the production of glass fiber requires hightemperature melting furnaces, heated bushings, and special crucibles.

Within the last decade, a number of patents have issued and otherliterature published describing various polycrystalline,microcrystalline, or non-vitreous fibers and other shaped articles ofrefractory metal oxides made by various non-melt processes, such as bydrying films of solutions of oxygen-containing metal compounds, ordrying organic polymeric bodies, such as cellulose or rayon, impregnatedwith such a solution, or by extruding and drawing, or spinning, viscousfluids of such materials into fibers, followed by heating to removewater and organic material and by firing at high temperatures to producea refractory article. A recent review of the state of the art onpolycrystalline inorganic fibers appears in Modern Composite Materials",edited by Brautman and Krock, published by Addison-Wesley Pub. Co.,Reading, Mass. (1967). An earlier article describing commercial zirconiapolycrystalline fibers is found in Ceramic Industry Magazine, April,1965, p. 120, and a relatively recent developmental product bulletindescribing yttria-stabilized zirconia fibers and textiles in BulletinNo. GER-101, April, 1968, of the Union Carbide Corp., N. Y. U.S. patentswhich have issued in this area are U.S. Pat. Nos. 3,082,099 (Beasley etal.), 3,180,741 (Wainer et al.),

3,311,481 (Sterry et al.) 3,311,689 (Kelsey), 3,322,865 (Blaze) and3,385,915 (Hamling).

The above-cited published literature and patents describe various metaloxide refractory materials and their preparation, including shapedarticles, such as fibers of zirconia. It is well known that calcined,unstabilized zirconia exists in different temperature dependentcrystalline modifications. Monoclinic zirconia is the form that isstable up to about 1,000 to 1,150C. From 1,000 to 2285C., the stableform is tetragonal, while above 2,285C. only the cubic form is stable.1f monoclinic zirconia is heated at about 1, l 50C., it undergoes atransformation to the tetragonal form, this transformation beingaccompanied by a radical volume contraction (about 9 percent), and oncooling it transforms again, and shaped articles, such as fibers, areweakened or broken when cycled through the 1,000 to 1 150C. temperaturerange. Thus, in all zirconia materials which are to be made or which areto be used or fired at temperatures above 1,000C. it has been necessaryto stabilize the crystal structure against the reversible transformationfrom the room temperature monoclinic form to the high temperaturetetragonal form.

Stabilization of zirconia has been sought by the use of certainstabilizing materials or additives, such as calcia, magnesia, neodymia,and yttria. Small stabilizing amounts of silica have also been used tostabilize zirconia in its-cubic form and produce non-vitreous fibersconsisting bf 5-1 5 weight percent SiO and -85 weight percent ZrO asdisclosed by U.S. Pat. No. 3,082,099. U.S. Pat. No. 3,31 1,481 disclosespolycrystalline refractory fibers of high moduli prepared by fiberizingan aqueous solution having a viscosity between 15,000 and 40,000 cps andcontaining zirconium diacetate, silica, and acetic acid, and drying andthen firing the fibers at successively higher temperatures, including1,8002000F. for 2 hrs. Zircon (zirconium silicate, having the formula ZrSiOQ, sometimes written as (ZrO2-SiO is of course a chemical compoundrather than a mixture of zirconia andsilica in their free or uncombinedforms, and it has a tetragonal crystalline form or structure, andarticles made of zircon, though strong, are not flexible.

Inorganic or metal oxide fibers and other shaped articles are stillrelatively in the early stage of development and hightemperaturetechnology today has a need for a relatively inexpensiverefractory product with desirable physical properties that can bemaintained at high temperatures.

Briefly, the refractory fibers of this invention comprise a solidhomogeneous mixture comprising microcrystalline zirconia and amorphoussilica, each of these components being present in theiruncombined orfree form as separate ZrO and S102 phases. As such, these fibers cannotbe classified either as polycrystalline or as amorphous, but rather area mixture of these two types of materials. Such fibers preferably havethe zirconia crystallites predominantly present in the tetragonal form(readily discernible by its characteristic X-ray diffraction pattern) asa result of firing the article in air at about 900 to 1,150C., or havethe zirconia present in another crystalline form, such as cubic, whichis irreversibly transformable to said tetragonal form when so fired.Such fibers are refractory, essentially chemically resistant,transparent, and strong. The fibers have significantly high elasticmoduly but yet low enough, e.g. below 25 X 10 psi, so as to be quiteflexible. (The term fiber" is used herein in the sense of a monofilamentwhich has a length-to-diameter ratio greater than 500 and as such can bethough of as a continuous fiber or one of indeterminate or infinitelength.)

The above-described refractory fibers in their green or unfired form canbe made by shaping and dehydrative or evaporative gelling, e.g. byextruding, drawing, spinning, or blowing, or combinations thereof, aviscous concentrate of an aqueous solution of an oxygencontainingzirconium compound, such as zirconium diacetate containing colloidalsilica. The shaped green fibers are then heated to remove further water,volatilize or decompose organic material, and burn off carbon, andconvert the fibers into a monolithic refractory comprising theabove-described mixture of microcrystalline Zirconia and amorphoussilica.

The refractory fibers of this invention can be used for a variety ofpurposes, particularly where high temperature stability is desired orrequired. For example, fibers of such refractory material can befabricated into woven, felted, or knitted textiles and used forheatresistant upholstery or clothing, and for other purposes wherethermal stability is desirable or required. Such articles can befabricated into textiles which are brilliantly internally colored withinorganic colorants and used to make decorative clothing, draperies,wall covering, and the like. Such articles can also be used asreinforcement for plastic, elastomeric, metal, or ceramic composites andas sound suppression material or as filtering or adsorptive material.The fibers also can be l,075C. fibers comprising 1ZrO :1SiO2:0.06 0.06

FIG. 3 represents transparent fibers, photographed at 50X withtransmitted light, obtained by firing at 1,000C. fibers comprisinglZrO2:lSiO and 50X with transmitted light, obtained by firing at 1,200"

Q'BOWC- fihsrss nzar s qs 139M510,

The composition from which the refractory product of this invention canbe formed is a two-phase system comprising an aqueous mixture of acolloidal dispersion of silica and an oxygen-containing zirconiumcompound which can be calcined to zirconia. The silica component ispreferably preseent in the amount of at least 0.5 mole and less than 3moles, per equivalent mole of ZrO2 present. Generally, the twocomponents or materials (the sole essential refractory precursorconstituents) are present in the mixture in amounts sufficient toprovide an equivalent ZrO :SiO mole ratio in the aqueous dispersion inthe range of 2:1 to 1:3, preferably 1.5:1 to 1:2,and more preferably1:1. Mole ratios higher than 1.5:1 generally will result in greenarticles which are difficult to fire to 'a transparent, clearrefractory, while solutions withmole ratios lower than 1:2 are generallydifficult to fiberize into green fibers. The particular .mole ratio ofthese components in the refractory product will be the same as that ofthe aqueous dispersion, e.g. a 1ZrO :1SiO2 refractory article isproduced from an aqueous dispersion having an equivalent mole ratio oflZrO2:1SiO

Th e dispersion can be prepa red by simply admixing an aqueous colloidaldispersion or aquasol of silica with an aqueous solution of thezirconium compound, preferably by adding the dispersion of silica to thelatter solution with mixing in order to obtain a uniform dispersion withformation of a gel, floc or precipitate. The pH of the resultingdispersion will be inherently low or on the acid side if the zirconiumcompound is an acid salt, such as zirconium diacetate, e.g. 3 to 5 andtypically 3.5 to 4, and though a heat fugitive acid, such as aceticacid, can be added to maintain a low pH and prevent premature gelling,such acid addition isnt necessary and may be undesirable in that thesubsequently formed green article when fired to form a refractory maytend to be black or gray rather than water-clear, depending on thefiring schedule or conditions. It is also within the scope of thisinvention to add heat fugitive organic viscosifying or thickening agentsto the dispersion where the particular aqueous solution of the zirconiumcompound is a solution of an inorganic zirconium compound like zirconiumsulfate or zirconium oxychloride, such viscosifyin g agents, e. g.polyvinylpyrrolidone, polyvinyl alcohol, methylcellulose, and glucose,being oxidized and removed during the firing of the green articlesproduced from such dispersions.

The zirconium compound can be used in the form of an aqueous solution ofa suitable organic or inorganic acid water-soluble salt, such as thezirconium salts of aliphatic or acyclic monoor dicarboxylic acids havingdissociation constant of at least 1.5 X 10*, such as formic, acetic,oxalic, maleic, adipic, itaconic, citric, tartaric, and lactic acids,and their halogenated derivatives such as chloroacetic acid, thezirconium diacetate being presently preferred because of itscompatibility with colloidal silica and commercial availability andrelatively low cost of its aqueous solution. An especially usefulaqueous zirconium diacetate solution which can be used in this inventionis one that is commercially available (see National Lead Co., TamDivision, Bulletin D-64) with a pH of 3.8 to 4.2, a specific gravity ofabout 1.33, and an equivalent of 22 weight percent ZrOZ, the zirconiumsalt having the formula H ZrQ;( Cl-l COO and sometimes named asdiacetato zir coiiic acid. rysiesri'sar'gsmc zirconium salts which canbe used are zirconium sulfate, zirconium nitrate, zirconium oxychloride,zirconium carbonate, and the like. Zirconia itself can be used in theform of an aquasol or aqueous colloidal dispersion of Zirconia, and whenmixed with the silica aquasol will form an aqueous dispersion ofcolloidal ZrO as well as colloidal SiOg. Since hafnia is commonlyassociated in nature with i'i'rcd'iii'a', corn rnerci allyavailablezircofiiu rn compounds or aqueous solutions thereof nonnally willcontain about 1 weight percent equivalent hafnia, and this oxide willnormally be present in the refractory products of this invention in acorresponding small or trace amount. Other impurities may be present inthe refractory article, but the total amount of such impurities will beless than 1 weight percent.

The silica aquasol or aqueous dispersion of colloidal silicab6,@531..Wiih.SiQzQQPEQIIIIfliQHiQf.1..9.59

weight percent, preferably to 35 weight percent, the latter concentrateddispersions being preferred because of their commercial availability andbecause if used, the amount of water that has to be removed from theresulting mixture in order to viscosify it will be less than if a moredilute dispersion is used. However, the colloidal silica can be used inthe form of an organosol, the silica being colloidally dispersed in suchwatermiscible, polar organic solvents as normal or isopropyl alcohol,ethylene glycol, dimethylformamide, and various Cellosolve glycol ethersas methyl Cellosolve (2- methoxyethanol). The size of the colloidalsilica particles in the aquasols or organosols can vary, e.g. from 1 to100 m;;., but generally will be on the order of 5 to 30 m preferablyabout 10 to 16 mp.

Preferred aqueous colloidal silica dispersions which can be used in thisinvention are those sold under the trademark Ludox (see duPonts BulletinA-56681 on Ludox Colloidal Silica). Other useful silica dispersions areNalco 1030, Nalco D-2l39, and Syton 200 (the latter being less preferredbecause of the tendency for the dispersion to foam excessively duringconcentration). The table below sets forth the properties of varioustechnical grades of aqueous colloidal dispersions of silica which can beused. In some cases it may be desirable to filter the silica dispersionto remove extraneous solids, bacterial growth and other materials.

The aqueous mixture of colloidal silica and zirconium compound can alsocontain various other oxygencontaining water soluble metal compoundswhich will impart a desired internal color to the final refractory uponbeing converted or oxidized to the corresponding metal oxide. Forexample, ferric nitrate can be added to impart a red to orange'to goldcolor, chromium diformate or chloride to impart a green color, cobaltace tate or chloride to impart a blue color, calcium formate to impart ayellow color, nickel acetate to impart a light yellow color, and copperchloride to impart a light green. The ferric oxide-containing refractorycan be reduced in a hydrogen atmosphere, the resulting re duced ironoxide or iron imparting a black color to the refractory and making itattractive to a magnet but not electrically conductive. (Such refractorycan be used to make composite ceramics useful in electrical equipment,such as coils, plates, rotors, magnetic circuitry, etc.) The amount ofsuch coloring additive will vary depending upon the tone of the color orhue desired, but generally will be an amount in the range of 0.5 to 10or 25 weight percent, preferably 1 to 5 weight percent expressed as themetal oxide, e.g. Fe 0 based on the combined weight of the zirconia andsilica in the refractory article. High amounts of some coloringadditives may result in refractory articles with less transparency andlower tensile strength, but even such lower tensile strength will beappreciably high, e.g. 100,000 psi 'or higher. Other coloring additives,even in small amounts, will cause opacity in the resulting refractory.Metal oxide precursors can also be added to adjust refractive index.

The aqueous mixture of colloidal silica and zirconium compound, asprepared, will be a relatively dilute liquid, generally containing about15 to 30 weight percent equivalent solids. For the preparation of shapedarticles such as fibers, it is necessary to concentrate or viscosifythis dilute liquid in order to convert it to a viscous or syrupy fluidconcentrate which will readily gel when the concentrate is formed into ashaped article and dehydrated, for example when the concentrate isextruded and drawn in air. The concentration step can be carried out bytechnique known in the art, which generally involve evaporation toremove large amounts of water and any volatile gases. Such evaporationcan be carried out at ambient temperature and pressures, but preferablyis carried out or finished under vacuum, such as that generated by awater aspirator. Such evaporation can be carried out in a flask whollyor partly submersed in a water bath having a temperature, for example of30 to C.; the liquid undergoing vacuum concentration in the flask will,of course, be cooler than this temperature, e.g. 0 to 10C. Suitableapparatus for concentrating the liquid is a Rotovapor flask partlysubmersed in a water bath and connected to a water aspirator. It is notnecessary to heat the dispersion at any time during the concentratingstep or after the desired viscosity is obtained; the preparation anddehydrative gelling of the concentrate can be carried out at ambientroom temperature. Sufficient concentration will be obtained when thesolids content is generally in the range of 40 to 55, preferably 45 to50 weight percent, and viscosities (Brookfield at ambient roomtemperature) in the range of 15,000 to 1,000,000 cps, preferably 45,000to 500,000 cps., depending on the type of dehydrative gelling techniqueand apparatus used and the desired shape of the gelled article. Too higha viscosity may be prohibitive for the particular equipment used toextrude the concentrate. The viscous concentrates are relatively stableand wont crystallize, and do not have to be freshly prepared for use,though the viscosity tends to increase on standing. The viscousconcentrates have capability of being diluted with water to dilutedispersions having the same appearance and apparent composition andproperties as the initial starting dispersion. If the dispersion isconcentrated to a viscosity which is too high to readily form desiredshaped articles, it can be diluted with water (and concentrated again ifdesired) to a workable viscosity. It, of course, is not concentrated tothe extent where the solubility of the zirconium compound is exceeded tothe point where macrocrystals are formed. Prior to dehydrative gelling,the concentrate can be centrifuged to remove air bubbles and/or filteredto remove extraneous solid material, bacterial growth, etc. Theparticular solids content or viscosity used for dehydrative gelling willbe dependent on the particular apparatus and conditions used to formshaped articles form the viscous concentrate. For example, when theviscous concentrate is extruded under pressure, e.g. 50 to 1,000 psi.,using a conventional stainless steel spinnerette, with a plurality oforifices (e.g. 15 to or more 1 to 10-mil orifices), such as used in therayon industry, the viscosity of the concentrate should be such thatfibers are formed in a continuous manner without blocking or plugging ofspinnerette orifices or breaking of the extruded fiber as it is formedor during handling, e.g. during spooling.

The viscous concentrate can be extruded through orifices and allowed tofall in air by the force of gravity or drawn mechanically in air bymeans of rolls or a drum or winding device rotating at a speed fasterthan the rate of extrusion, or the concentrate can be extruded throughorifices from a stationary or rotating head and blown by parallel,oblique of tangential streams of air, such as in the making'of cottoncandy, the blown fibers being collected on a screen or the like in theform of a mat. Any of these forces exerted on the extruded fibers, e.g.gravity, drawing, or air streams, cause attenuation or stretching of thefibers, reducing their cross-sectional area by about 50 to 90 percentandd increasing their length by about 100 to 1,000 percent and serve tohasten or aid the drying of the fibers.

The dehydrative gelling of the article is most conveniently carried outin ambient air, though heated air can be used if desirable or necessaryto obtain fast drying. The relative humidity (RH) of such air should notbe too high, since large amounts of moisture will cause the gelled orshaped green articles to stick together. Generally, relative humidity inthe range of 20 to 60 percent can be used, at temperatures of 60 to 85F.If the humidity is high and must be tolerated, compensations can be madeby using a concentrate with a higher viscosity, extruding at a lowerrate, using a lower drawing rate, using a smaller extrusion orifice,exposing the green articles to a heat lamp as they are formed, and/orincreasing the distances between the extrusion orifice and the pointwhere the individual extruded articles come into contact. Sizing of thegreen fibers before they come into contact, e.g. to form strand ofmultifibers, will also lessen their tendency to stick together in a highhumidity atmosphere. Where a size is used, the extruded fibers can bemechanically drawn over a size applicator, like that used in the textileindustry, and a conventional heat fugitive size or lubricant, such as anoil, applied. Heat lamps or the like can be used to volatilize the sizeso as to avoid combustion of the size when the green articles are fired,such combustion tending to cause overheating of the articles (i.e., thetemperature caused by combustion may be higher than the desired firingtemperature). The size may also require longer firing to completelyremove it from the fired article. On the other hand, if the relativehumidity is too low, eg 10 to percent, or lower, the green articles maydry too fast and they will tend to break or fracture during spinning orhandling before they can be fired. Low humidity conditions can becompensated for by extruding at a faster rate, drawing the extrudedarticle at a faster rate, using larger extrusion orifices, decreasingthe distance between the orifices and the point where the articles come-into contact, and/or using concentrates with lower equivalent solidscontent or lower viscosities. Air currents should be minimized becausethey may cause the individual extruded articles to come into contactbefore they are sufficiently dry. In any event, the extruded orotherwise gelled articles should be made or handled under conditionswhich will prevent or minimize their contact with one another beforethey are sufficiently dry.

Further detail on the shaping of articles from the viscous concentratewill be omitted here in the interest of brevity since such shapingprocedures are well known, reference being made to the aforementionedcitations, including Chapter 8 of said Modern Composite Materials textwhich illustrates and describes apparatus which can be used in thisinvention to form fibers from viscous concentrates.

The fibers in the green or unfired gel form, as well as other shapedgelled articles fabricated from the viscous concentrate, generallycomprise about 60 to weight percent equivalent solids and are dry in thesense that they do not adhere or stick to one another or othersubstrates and feel dry to the touch, but they still do containsubstantial amounts of water and organic material, e.g., 20 to 40percent, and it is necessary to dry and then heat or fire the articlesin order to remove further water and organic material and convert thearticles into refractory articles comprising a mixture of zirconia andsilica. The terms dehydrative gelling or evaporative gelling, as'usedherein, therefore does not mean that all the water in the shaped articleis removed and does not mean the crystals are formed. Thus, in a sense,this step can be called partial dehydrative gelling. It may be noted atthis point that the shaped articles in their green form are transparentand clear under an optical microscope; and unless coloring additives areincluded in the viscous concentrate, they appear to look like colorlessglass fiber. These green fibers, as well as other solidified gelarticles (such as microspheres and flakes) of this invention in theirgreen form, are composed of a dispersion of amorphous silica particles(in colloidal size) in an amorphous matrix. This was established byelectron micros copy of a dry thin film of the initially prepareddispersion of colloidal silica in aqueous zirconium diacetate, the filmhaving been prepared by dipping a nickel or platinum screen in theinitially prepared dispersion (in dilute or unconcentrated form) andthen drying the wet screen in air.

In order to remove the balance of water and organic material from thegreen or solidified gel articles, they are heated in an electricfurnace, kiln, or the like in air, oxygen, or other oxidizingatmosphere, at moderately high temperature of about 5005 50C., or evenas high as 900C. if desired. The green fibers can be heated in the formof individual fibers collected in an irregular or random order, or,preferably, in the form of a spool of strands (a plurality of untwisted,parallel-aligned fibers), or in the form of hanks (a bunch of strands),or they can be chopped in the form of stables and fired in that manner.Also, the green strands or fibers, preferably twisted in the form ofyarn, can be woven to form a cloth and heated in the latter form toremove water and undesired constituents. in firing the green articles,care should be exercised to avoid ignition of the articles, so as toprevent or minimize the formation of opaque, fragile articles. If thegreen articles are not to be fired immediately or soon after theirformation, they preferably should be stored in a relatively dryatmosphere to prevent them from picking up moisture and stickingtogether.

As confirmed by thermal gravimetric and differential thermal analyses,the heating step volatilizes the balance of the water, decomposes andvolatilizes organic material, and burns off carbon, the resultantarticle being a carbon-free monolithic refractory. This heating stepalso causes some shrinking of the article, the amount of linearshrinkage being generally 25 percent or more, and the volume shrinkagebeing generally 50 percent or more. However, the shape of the articleduring firing remains intact; for example, fibers when so fired arestill of essentially continuous length. Rather than firing the greenarticles in air to remove water and .organic material, they can beheated in an autoclave in an inert atmosphere (e.g., 100 to 2,000 psi,helium, argon, nitrogen), for example at 300 to 500C., in order toincrease their porosity. Then, they can be refired in air to removecarbon, e.g. at 500 to 900C, and convert them into a refractory free ofcarbon.

The refractory materials fired at 500 to 900C. have desirable propertiesand may be used as such without further heating. Fibers so fired arestill transparent and clear under an optical microscope and willgenerally have densities in the range of 3.0 to 3.75 g/cc., (or as lowas 2 g/cc. if autoclaved in an inert atmosphere) with diameters in therange of 10 to 40 u, tensile strengths (at break) of 25,000 to 160,000psi, usually 35,000 to 75,000 psi, and elasticity moduli (Youngsmodulus) in the range of 5 to 18 X psi, usually 7 to 10 X 10 psi.

Electron microscopy with a resolution of about 10 Angstroms reveals thatthese 500 to 900C.-fired refractory articles comprise a homogeneousmixture of discernible microcrystals and amorphous material. X-raydiffraction identifies zirconia as the sole diffracting crystallitespecies with crystallite sizes in the order of 200 to 400 Angstroms(based on estimates from line broadening), such crystallites beingrandomly mixed in no preferred order with the silica component presentin its amorphous form.v The X-ray diffraction pattern for the zirconiacrystallites was found to be consistent with the ASTM X-ray diffractiondata card 7-337 for cubic zirconia, though because of the smallcrystallite sizes present, and the consequent line breadth in thediffraction pattern, it was difficult to rule out the presence oftetragonal zirconia or a mixture of cubic and tetragonal zirconia. Theserefractory articles can be further characterized as articles which whenfired at higher temperatures, as described in the next paragraph,contain tetragonal zirconia as the major crystalline component in theso-fired refractory.

Though the refractory articles resulting from the firing at 500 to 900C.have useful physical properties and can be used as such, suchproperties, particularly tensile strength, elastic moduli, and density,can be enhanced by firing the shaped articles in air or other oxidizingatmosphere at higher temperatures a sufficient period of time such thatessentially or substantially all of the zirconia component is present inits tetragonal form and positively identifiable as such by X-raydiffraction. This tetragonal form is surprisingly irreversible andmaintained when the so-fired articles are cooled to ambient temperature.Generally, these high temperatures will be in the range of 900 to1150C., preferably 950 to 1050C. These temperatures, of course, are wellbelow melting or fusing temperatures. Longer firing at the lowtemperatures in these ranges generally will be equivalent to shorterfiring at the higher temperatures in these ranges, so far as formationof zirconia crystallites in predominantly the tetragonal form isconcerned. These higher temperatures can be approached from roomtemperature by gradually or incrementally raising furnace temperature,or the articles can be placed in their dried green form or their 500 to900C.-fired form directly in a furnace which has previously been heatedto the desired temperature of 900 to 1,150C. However, when greenarticles are directly fired at a particular temperature in this range,ignition may occur and cause opaque articles to form if the firedarticles are too closely packed in the furnace. Variations in firingschedules will become apparent to those skilled in the art. Dried greenarticles can be heated to or at 500-550C. for a period sufficient toconvert the article to a refractory and then can be placed directly in al,000C.-fumace and heated for a period sufficient to form a refractoryarticle with the desired mixture of tetragonal zirconia and amorphoussilica, usually 30 minutes to 1 hour. Prolonged heating at l,000C., orslightly above this temperature, will not adversely affect therefractory, though if the final firing temperature is in the range ofl,050 to l,150C., it should be held at such temperatures for only ashort time, e.g. 15 to 20 minutes or less, to avoid formation ofsubstantial amounts of zircon. In order to obtain a uniform firedproduct, green articles to be fired should not be too closely packed asto cause carbon to be trapped and retained and should be placed in thatportion of the furnace where temperature control is assured. Differentsubstrates or saggers on which the products are disposed during firingmay result in products having somewhat different appearances (e.g.,translucent rather than transparent), though generally the same desiredphysical properties and microstructure will be obtained with any heatstable substrate. Here, again, in firing the products of these highertemperatures, they can be in the form as produced by the dehydrativegelling step or in a modified form, e.g., strands, spools, cylinder offibers, hanks, tows, cloth, yarn, etc.

At about 900C., a significant microstructural change occurs. X-raydiffraction analysis of 900 to 950C.- fired fibers definitelyestablished the presence of tetragonal zirconia with larger crystallitesizes. Electron microscopy with a resolution of about 10 Angstroms of900 to 1,000C.-fired dry thin films of the initial unconcentrateddispersion showed a consolidation or diffusion together of adjacentgrains of the same species to form larger curvilinear areashomogeneously mixed together in a mottled array. Some of the areaschanged from black to grey, or vice versa, due to diffraction of thecrystalline grains (apparently ZrO as the incident angle of the electronbeam was changed, and other grains or areas maintained their tone (lightgrey to white) indicating a non-diffracting amorphous phase, apparentlysilica, this latter phase looking like a matrix in high silicarefractories, e.g. 1ZrO :1SiO

The structural tmasformation is coincident with a significant increasein tensile strength and elastic moduli. Such tetragonal zirconia, andthe coincident strength, flexibility, transparency, and clarity, areretained when the refractory products are cycled at temperatures up to900 to 1,150C., and thus will not mechanically fail due to phasetransformation or thermal shock. When fired at 900 to 1,150C. for thedesired duration, X-ray diffraction reveals that these refractorymaterials have crystallites in the order of 400 to 1,000 Angstroms,based on estimates from line broadening, and that such crystallites areessentially or predominantly all tetragonal zirconia (the X-raydiffraction pattern found for this species of crystallite beingconsistent those with ASTM Card 14-534) with a very minor amount, ifany, of monoclinic zirconia present (the presence of this latter type ofzirconia erystallite being confirmed by an X-ray diffraction patternconsistent with ASTM Card 13-1307), and no evidence of any cubiczirconia being present. Further,- both X-ay diffraction and electrondiffraction establish that the silica component is undetectable andapparently present in its amorphous state in admixture with the zirconiacrystallites. Thus, these refractory products cannot be properlyclassified as polycrystalline, microcrystalline, or amorphous, butrather as a mixture of crystalline material (zirconia) and amorphousmaterial (silica). (An amorphous material is generally accepted asmeaning one which does not have any crystallites that are discernibleunder X-ray diffraction analysis.) Such refractory articles arelikewise'transparent and clear when viewed under an optical microscope.They can also be characterized as monolithic and zircon-free.

- The refractory fibers of this invention fired at.900'? to 1,150C. willgenerally have densities in the range from 3.0 to 4.3 g/cc. (or as lowas 1.5 g/cc if prepared from green articles which were initiallyautoclaved in an inert atmosphere to increase porosity), with diametersin the range of to 40 a, tensile strengths (at break) in the range from50,000 to 250,000 psi. and higher (e.g. to as much as 500,000 psi. insome instances), usually at least 100,000 psi. and typically 150,000 to250,000 psi., and with elasticity moduli in the range of 7 to X 10 psi.,usually 10 to 18 X 10 psi., these various properties varying with therelative ZrO :SiO mole ratio in the fibers, the presence of coloringadditive, porosity, and the temperature and duration of firing. Therefractory products also have high melting points, high purity, lowthermal conductivity, and are generally resistant to common chemicals,e.g. acids. For example, l,000C.-fired fibers of 1ZrO :lSiO having anaverage tensile strength of 182,000 psi., when soaked 34 days inconcentrated hydrochloric acid were found to lose .only about 7 percentof their strength and they were ter-clear.

' If desired, these refractory articles can be heated for extendedperiods (e.g. 2 to 12 hours or more) at temperatures above 1,000C., e.g.at temperatures of 1,050 to 1250C. or higher, to convert the refractorymaterial to one which contains zircon (ZrSiO such refractory articlescontaining various amounts of zirconia (present usually in itstetragonal and monoclinic forms) and silica (present in its amorphousand cristobalite forms) in their uncombined forms. However, refractoryarticles containing significant amounts of zircon have significantlyreduced strength and flexibility, and as such are less preferredrefractory materials. Depending, on how high the further firingtemperature is and its duration, the refractory may contain substantialamounts of zircon (i.e., with X-ray diffraction lines having a relativeintensity of 100) and will be translucent or more often opaque, and willbe brittle and fragile. The extremely high temperatures will evenconvert the refractory to one consisting essentially of just zircon ifthe ZrO and SiO components are present in equimolar amounts. (Whenzircon and cristobalite silica have been found, their X-ray diffractionpatterns were found to be consistent with ASTM Cards 6-266 and 11-695,respectively.)

The'nature of the refractory articles of this invention and the resultsobtained by firing at elevated temperatures are illustrated in theaccompanying drawing. FIG. 1 illustrates what the preferred fibers ofthis invention look like when examined under an optical microscope withtransmitted light, these particular illustrated fibers being thoseprepared in Example 17 (Run 5) fired at 1,000C for 1 hour, and composedof a mixture of ZrO and SiO in a mole ratio of 1:1 and containing 0.06

mole of ferric oxide to impart a gold color. These fibers are clear,transparent, essentially continuous in length and round in cross-sectionin appearance as well as strong and flexible, X-ray diffraction analysisrevealing only ZrO crystallites, the relative intensity of thediffraction lines attributable to tetragonal ZrO being 100 and thoseattributed to monoclinic ZrO being 10. Note especially in FIG. 1 thetransparency of these fibers: where one fiber intersects and overlaysthat of another, the diffracted outline of the fiber below can besharply seen through the top fiber, as indicated for example byreference number 1. When such fibers, however, are fired at 1,050 and1075C. for 1 hour (as described in Example 17, Runs 7 and 8), theybecome opaque black under transmitted light and rust red when similarlyviewed with the unaided eye or with oblique light under a binocularmicroscope, and have the appearance depicted in FIG. 2. The fired fibersillustrated in FIG. 2 were weak and fragile and composed of asignificant amount of zircon (with X-ray diffraction lines having arelative intensity of to as well as tetragonal and monoclinic ZrO FIG. 3depicts the appearance of fibers similar to those of FIG. 1, except thatthey were composed of 1ZrO :lSlO (and no Fe O and resulted from firinggreen fibers from room temperature to 1,000C., as described in Example10. When these fibers were retired to 1,200 to 1300C. for 2 hours, theyhad the opaque black appearance under transmitted light (or opaque whiteunder oblique or incident light) depicted in FIG. 4 and were brittle,breaking into the small lengths shown in that figure during handling inthe course of mounting them for microscopic examination. The1,000C-fired fibers of FIGS. 1 and 3 when broken orfractured foranalysis with a razor blade from a tow of such fibers and viewed with ascanning electron microscope at 300x, showed the fibers to have a roundor circular cross-section, some of the broken ends of the fibers havingthe appearance of a conchoidal fracture, such fractures being common tononcrystalline or amorphous structures.

In describing refractory products of this invention as transparent, thisterm means that the particular article in question, when viewed under amicroscope has the property of transmitting rays of light, so thatbodies beneath the article, such as bodies of the same nature as thetransparent article, can be clearly seen through the transparentarticle, the outline, periphery or edges of bodies beneath being sharplydiscernible. Opaque articles, on the other hand, are those which areimpervious to light and bodies beneath are obscured by opaque articleand cannot be seen therethrough. The translucent articles are thosewhich fall between transparent and opaque, and though translucentarticles have the property of transmitting light to some degree, andtherefore are somewhat or partly transparent, bodies beneath can be seenin a diffuse manner rather than in a clearly distinguishable or sharpmanner. Sometimes, because of vagaries in firing, an article or productmay be a mixture of these various types of products, though generallyone will be present in a predominant amount, indicative of the truenature of the mixture, the other products present in minor amountshaving their particular appearance due to incomplete firing at thedesired temperature or due to overheating because of hot spots in thefurnace.

Articles of this invention are preferably those which are transparentthough for some particular applications, for example where the productis used as a reinforcement for composites, transparency will not beimportant. The transparent quality of a refractory product of thisinvention is coincident with other desirable properties, such asstrength and flexibility and the presence of zirconia, and thustransparency can be considered in a sense as a gross measure of thequality of the refractory product. In some applications of therefractory products of this invention, e.g. where a fiber or bundle offibers are .used in fiber optics or where microspheres are used inreflective sign surfaces, transparency will be of special importance.

Flexibility is another characterizing property of some of the refractoryfibers of this invention. Flexible fibers in the form of monofilamentsor in multifiber forms, e.g., threads, strands, yarns, rovings, tows,etc., are capable of being handled and fabricated, for example asflexible woven textiles or cloths, without breaking or otherdisintegration when bent or twisted, and in this application flexibilitymeans that a plurality of fibers, e.g. 100, in the form of a tow orstrand can be twisted to form a yarn or tied in the form of a figure 8knot without breaking. Such flexible fibers will generally have elasticmoduli less than 20 X 10 psi., and usually in the range of to 18 X psi.,moduli higher than 20 X X 10 psi. being relatively stiff. It isntessential in this invention to make the fibers with fine diameters inorder to obtain flexibility, since fibers having diameters as large as uhave been prepared with excellent flexibiliy.

The refractory product of this invention is generally particularlyuseful where high temperature stability or refractoriness is desired orrequired, for example up to about l,000 to 1,1 00C. Above suchtemperatures, the refractory products of this invention generally willbegin to lose strength, flexibility, and transparency, coincident withthe formation and crystal growth of zircon and the appearance ofmonoclinic zirconia in X-ray diffraction analysis at room temperature.However, where such loss in properties are not important for theparticular application of these refractory products, they can beemployed in such applications since they will retain their solid stateto temperatures as high as about l,675C. and higher, the refractoryproducts of this invention having an equivalent mole ratio of ZrO :SiOof 1:1 becoming completely molten only at about 2,400C. The refractoryproducts of this invention can be employed alone or per se in variousapplications in the form in which they are obtained as fired, or theirphysical form can be modified, e.g., comminuted or pulverized to form apowder, or in their form as prepared or as modified they can be mixed orcoated with or bonded to other materials, e.g. composite matrixmaterials.

The refractory fibers of this invention are particularly useful infabricating woven, felted, knitted, and other types of textiles such asbraids. Such textiles generally will have the same properties, such ashigh strength,

flexibility, refractoriness, and chemical resistance, as the fibers fromwhich they are made. The internally colored refractory fibers will findparticularly useful application in decorative fabrics, such as used inclothing, upholstery, wall covering, etc. Fibers or yarns of thisinvention of different colors and/or composition can be used together inmaking fabrics with decorative designs. Some of these fibers, such asthose containing ferric oxide as an internal colorant or additive, arecapable of being branded to form designs thereon of different color.Fibers or yarns of this invention can be plied or interwoven with fibersof other materials, such as metal fibers, silica fibers, carbon,graphite, Teflon polytetrafluoroethylene or fiberglass, if desired.Woven cloths made from the refractory fibers can be firmly bonded aswall covering to various substrates. For example, such cloths can bebonded with molten glass, or refractory cements such as zircon, aluminumoxide, phosphates, and silicates, to aluminum or other metal substratesand used as the interior walls of airplanes. The woven cloths (or mats)can also be used as layups in plastic, metal, or ceramic laminates.

The refractory fibers of this invention can be used in the form offabrics, mats and batting as lightweight acoustical or thermalinsulation for high temperature equipment, such as resistance andinduction furnaces, and for purposes of heat shielding or reflecting,such as heating mantles and thermal curtains.

In their porous form, the refractory fibers are useful in filtering oradsorption applications, for example a filter to remove solids for hotgases, e.g. particulate matter from cigarette smoke, or as achromatographic column packing to selectively separate or resolveliquids or gases, or as catalysts or catalyst supports.

Another particularly useful application for the refractory products ofthis invention is that of reinforcement for structural plastic,elastomeric, metallic, or ceramic composites, especially thosecomposites used in high temperature environments or even hyperthermalenvironments found in the aerospace industry, and in ablativeenvironments.

The refractory fibers can be used to form fiberreinforced plasticcomposites and fiber-reinforced metal matrix composites. The matrixmaterials which can be so reinforced include any of those heretoforeused in making such composites, such as those disclosed in theabove-cited Modem Composite Materials text. The plastics may be eitherof the thermosetting or thermoplastic types. Representative plasticswhich can be used include epoxy resins, polyester resins, acetal resins,acrylics, especially methyl methacrylate polymers, amino resins,especially ureaformaldehyde, and melamine-formaldeyhde, alkyds,cellulosics, especially ethyl cellulose, cellulose acetate, andcellulose proprianate. fluorocarbons, furanes, polyurethanes, phenolics,polyamides, polycarbamates, vinyl aromatic such as styrene, polyolefins,especially polyethylene, and the like. The refractory fibers used asreinforcement for such plastics serve to strengthen shaped articles madefrom such plastics. The techniques which can be used in incorporatingthe refractory products of this invention as reinforcements in plasticmatrices are well known, see Handbook of Reinforced Plastics, by Oleeskyand Mohr, Reinhold Pub. Corp., N. Y. (1964).

Metal matrix composites have had generally only limited applicationheretofore, one major reason being the lack of reinforcement materialswhich will withstand the elevated temperatures encountered inprocessing, e.g. casting and sintering temperatures. The refractoryfibers of this invention, because of their thermal stability, strength,flexibility and other properties, are useful as reinforcements for metalcomposites, such as shaped or cast articles made of aluminum, copper,magnesium, nickel, titanium, etc. Here too the prior art methods ofincorporating reinforcements in metal matrix composites can be used,reference being made to Fiber- Strengthened Metallic Composites, ASTMSpc. Tech. Pub. No. 427, published by the American Society for Testingand Materials, Phil, Pa. (1967).

The refractory products of this invention can also be used asreinforcement for ceramic composites, such as silica, glass, aluminumsilicate, and other inorganic materials, such reinforced ceramics beingin the form of blocks, paper, and other shaped articles used in hightemperature environments.

The refractory fibers of this invention can also be used as abrasionresistant and/or reinforcing agents for elastomeric materials, such asrubber, e.g. natural rubber, styrene-butadiene rubbers (SBR),acrylonitrilebutadiene rubber (NBR), and neoprene (WRT),'for examplewhere such rubbers are used in making passenger-car or truck tires.

The objects and advantages of this invention are further illustrated inthe following examples, but it should be understood that 'the particularmaterials used in these examples, as well as amounts thereof, and thevarious conditions and other details described, should not be construedto unduly limit this invention. In these examples, the various Ludox,Nalco, Syton colloidal silica products recited and used are thoseidentified and described hereinbefore. The aqueous zirconium diacetatesolution recited in these examples is the commercially available producthereinbefore described, having an equivalent of 22 weight percent ZrOThe viscosities recited are Brookfield viscosities measured at ambientroom temperature. Tensile strength data on fibers was obtained byvertically suspending a single fiber between two mounting blocks andadding weights of know value tothe lower block until the fiber broke.The elastic moduli values reported are those obtained according toMethod FMT-ll described in the U.S. Air Forces Tech. Rep.AFML-TR-67-l59, Sept.,

. 1967. Weight percent solids data was obtained by drying and firing inair to about l,000C. a sample of the dispersion. Where the nature of thevarious fired refractory products is described as being that observedwith an optical'or binocular microscope, the nature described is thatobserved under magnification with oblique light. The recitedidentification of crystallites by means of X-ray diffraction wasverified by comparing the observed pattemwith the particular ASTM cardreporting such pattern, the code numbers for these various ASTM cardsbeing recited hereinbefore. The X-ray data reported is that obtained atroom temperature with a General Electric Co. X-ray diffractioninstrument, XRD3, at 40 kv, ma, using a powder diffraction camera(Debye-Scherrer) with an effective film diameter of 14.32 cm. Unlessotherwise indicated,

- the sample were powdered samples exposed 0.5 to 2 hours to copper, K aradiation wavelength 1.5405 Angstroms, filtered through a nickel filter.Where cubic zirconia is reported in various X-ray analyses, the presenceof tetragonal zirconia should not be inferred as being necessarilyabsent, the line breadth in the diffraction pattern being such as to notpositively exclude such species, as mentioned hereinbefore.

EXAMPLE 1 Eighty grams of an aqueous colloidal dispersion of silica(Nalco 1030) were added with mixing to 225 g. of an aqueous solution ofzirconium diacetate to provide a colloidal dispersion of silica inaqueous zirconium diacetate having a pH of about 4, an equivalent ZrO:SiO mol ratio of 1:1, and an equivalent solids content of about 48 wt.percent. The resulting aqueous dispersion was slightly cloudy on mixingbut became clear when concentrated. Such concentration was achieved byplacing the dispersion in a rotating flask partially immersed in a3050C. water bath and rotating the flask while maintaining the contentsunder a vacuum (ca. 29 inches Hg) with a water aspirator, thisconcentration step being continued until the resulting concentratebecame viscous enough (about 50,000 to 70,000 cps) to enable fibers tobe pulled therefrom with a glass rod. The viscous concentrate was thencentrifuged for about 15 min. in a laboratory test tube centrifuge toremove bubbles. The resulting clear viscous concentrate was thenextruded under a pressure of psi into ambient air (ca. 22C.) thougha'gold-platinum spinnerette having six round 3-mil orifices. The fibersor filaments so extruded were essentially continuous and straight andwere drawn for about 3 ft. in the air and wound in a parallel fashion ona variable speed take-up drum covered with a sheet of polyester film,the speed of the drum being adjusted to exert a slight pull on theextruded fibers and hold them taut as they were extruded and wound,thereby effecting attenutation of the fibers. The green fibers so spunwere essentially dry on their surface, they did not stick together afterbeing wound, and were glossy, looking much like spun glass fiber, andunder a binocular microscope appeared water clear, transparent, smooth,round in cross-section, and straight. As wound on the drum in the formof a coil or winding, the fibers appeared white and glossy.

The coil was removed from the drum and fired inair by placing the samein an electric furnace (Temco Model No. FL-630), raising the temperaturefrom room temperature up to 550C., and holding at 550C. 2 hrs. Some ofthe'550C.-fired fibers-were further fired in air in said furnace forabout 1 hr. at 775C., and some of the 550C.-fired fibers were fur therfired in air at 1,000C.'overnight (about 12 hrs.). Some of the re-.sulting 775C.-fired fibers were fired in air to 1,050C. Some of thefibers fired to 1,050C. were fired in air at 1,050C. overnight. Some ofthe latter 1,050C.-fired fibers were then fired in air for about 48 hrs.at 1,100C. The 550C.-fired fibers had diameters of about 10-17 p. withtensile strengths in the range of 75,000 to 150,000 psi. The 775C.-firedfibers had diameters of about 15 p. with tensile strengths in the rangeof 95,000 to 155,000 psi. The fibers fired to 1,050C. had diameters ofabout 15 I1. and tensile strengths in the range of 90,000 to 160,000psi. The

550C.-, 775C.-, and l,000C.-fired fibers and those fired to 1,050C. allappeared transparent, glossy, smooth, round, and essentially continuousand straight under a binocular microscope. Those fibers which were heldovernight at 1,050C. had diameters of about 15 p. and tensile strengthsof 60,000 to 75,000 and exhibited a sharp extinction when examined witha microscope using polarized light, and exhibited some smallcrystallites or seeds, probably zircon. (Tensile strength values givenabove are those obtained at the moment the fibers being tested brokeunder the applied load.) All fibers, except the l,lC.-fired, wereflexible.

X-ray diffraction analyis of the 775C.-fired fibers revealed thepresence of exclusively ZrO crystallites, in the cubic form, the SiOcomponent being undetected and being apparently present in the amorphousstate. The 1,000C.-fired fibers did not break in handling, i.e. theywere strong, and X-ray diffraction analysis showed the presence of ZrOpredominantly in the tetragonal form (relative intensity 100) and to asmall extent in the monoclinic form (relative intensity 25), the SiOcomponent again being apparently amorphous, The 1,100C.-fired fiberswere found under X-ray diffraction analysis to comprise a mixture ofzircon (relative intensity 100), tetragonal ZrO (relative intensity 80)and monoclinic ZrO (relative intensity 40), the balance of the SiOcomponent, not combined in the form of zircon, again being apparentlyamorphous. When these l,l00C.-fired fibers were examined under anoptical microscope, they were found to be composed of opaque crystallineareas some of which were as wide as the diameter of the fibers (about p.with lengths up to about 5 times the diameter (i.e. about 75 a), theseopaque crystalline areas being separated by translucent milky areas.These 1 ,l00C.-fired fibers were very fragile and broke in handling.

The above results and evaluations of the various fired fibers show interalia that the strongest transparent fibers were those obtained by firingto a temperature where substantial amounts of stable tetragonal ZrO areformed, e.g. to 1,050C., and that firing for extended periods at 1,050C.and above (e.g. l,l00C.) reduced their strength and caused them tobecome opaque, such fibers no longer having tetragonal ZrO in a dominantamount but containing zircon crystallites and monoclinic ZrO insubstantial amounts.

EXAMPLE 2 A colloidal dispersion of silica in aqueous zirconiumdiacetate was prepared by adding 48 g. of colloidal silica (NalcoD-2139) to 112.5 g. of aqueous zirconium diacetate, the ZrO :SiO moleratio equivalent of the resulting dispersion being 1:1. The latter wasconcentrated using a procedure like that of Example 1 to produce aviscous concentrate having a viscosity of 120,000 cps. Green fibers werespun from this concenbinocular microscope with diameters of l5-20 p. Aportion of these 500C.-fired fibers were refired in air at 1,000C. for1% hrs., the resulting fibers being the same in appearance as before,with a modulus of elastieity ofover the range of 7 to 20 X 10 psi., andan average of 12 X 10 psi., and a density (determined by mercuryporosimetry) of 3.86 g./cc.

EXAMPLE 3 A colloidal dispersion of silica in aqueous zirconiumdiacetate was prepared in a manner similar to that of Example 1 using160 g. of an aqueous colloidal dispersion of silica (Ludox SM-l5), whichhad been filtered first through a Whatman No. 54 filter paper and thenthrough a 10 u Millipore filter paper, and 225 g. of the aqueouszirconium diacetate solution, the equivalent ZrO :SiO mol ratio of theresulting mixture being 1:1. The resulting dispersion or sol stayedclear as mixed and was concentrated in a Rotovapor flask under thevacuum of a water aspirator, the flask being rotated in a 3050C. waterbath for about 4 hrs., and then rotated overnight at ambient pressureand temperature. The resulting concentrate had a viscosity of about52,000 cps. and was spun in a manner like that of Example using aspinnerette pressure of psi. and a drum speed sufficient to wind eachfiber at 100 linear feet per minute. The resulting coil of green fiberswere fired from room temperature to 500C. and then held at 500C. for 20min. Another coil was similarly fired at 560C. for 20 min. The fibers ofboth fired coils were continuous and straight and could be twisted toform a yarn without breaking, and under a binocular microscope appearedwater-clear, transparent, glossy, smooth and round. Both of the coilswere then further fired at 1,000C. for 1 hr., yielding strong, flexiblefibers having a tensile strength at break of 250,000 psi and the sameappearance as those of the 500C.- and 560C.-fired fibers.

EXAMPLE 4 Viscous concentrate like that of Example 3, with an e qilivaTeht z r olfii mole ratio of l f 1 was similarly spun at 80 psi.,using a drum speed sufficient to wind each fiber at a rate of l 10linear feet per minute. Portions of the resulting green fibers werefired at different Table 1 Run Firing Density, g/ce Tensile strength,psi Modulus of elasticity, psi. X

1 from RT to 500C., 3.0 25,000-35,000 7.4 9.5

and 1.5 hr. at 500C. 2 S00C.fired fibers 3.45 74,00096,000 11.5 17.3

further fired for 1.5

hr. at 750C. 3 500C.fired fibers 3.74 130,000-145,000 13.5-18.3

further fired for 1.3 hr. at l,000C.

trate in a similar manner and fired in air from room These results showthe greater strength and modulus o f elasticity obtained by firing at1,000C., coin ci dent with the detection of tetragonal ZrO as comparedto firing at the lower temperatures of 500C. and 750C.

EXAMPLE Fibers spun in a manner like that of Example 3 from the sameconcentrate (with e ui'vsle'nfztogsioz mole ratio of 1:1) were fired inair at various temperatures. All of the fired fibers were flexible,continuous and straight, and under a binocular microscope they werewater-clear, transparent, round and smooth. The densities, tensilestrengths (at break), and modulus of elasticity values of the firedfibers were determined as well as their X-ray pattern. Results aresummarized in Table ll.

ing of fibers) was passed under one revolving roll and then over anadjacent, oppositely revolving roll, and the strand allowed to fallfreely 3 ft. to collect or piddle on a continuously moving belt ofaluminum stock (4 mil X 18 in.) moving at the rate of 1 ft./min. Theloaded aluminum belt passed under a 9 ft. long bank of heat lamps andinto a 6 ft. Globar electric furnace set at a temperature of 1,050F. andhaving a residence time of about 6 min. The strand on the moving beltturned brown in the first couple of inches of the furnace, then turnedblack and was essentially white in appearance at about the middle of thefurnace where Ta ble ll- Tensile Modulus of Apparent order X-raydiffraction Density Diameter, strength elasticity V of crysta l size,Zrl) phase, relative Run Firing g/cc p. psi psi X l0" by X-faY 'exam..Aintcnsity*" Cubic Tetra Mono 1 500C. for 1 hr. 3.30 l9-20 37,000- 6-l0200 400 I00 59.000 2 600C. for 1 hr. 3.49 18-20 45,000- 7-12 200 400 I00I Hm W 48,400 3 700C. for l hr. 3.49 l5-30 43,475- 9-ll 200 400 I0055,00 4 800C. for l hr. 3.60 17-19 58,000- 9-ll 200 400 V 100 l50,000 5900C. for 1 hr. 3.57 18-20 62,000- l2-l7 400 800 I00 l46,000 l000C. for4 3.65 lS-IB ll8,000- l3-l6 600 l000 100 l hrs.

175,000 7 l000C. for 4 3.85 l5-20 59,000- l0-l7 800 100 l hrs.

2l6,000 8 l000C. for 19 3.93 18-30 l02,000- l3-20 800 100 8 hrs.

160,000 9 l 100C. for l hr. 3.79 15-20 I 15,000- 15-17 800 lOO 6 Fibersample slipped out of mount during test at l70,000 psi; it was evidentlystronger than this value.

"X-ray diffraction of the 900C.-fired fibers indicated that the ZrO,crystallites in this run were either cubic or tetragonal; it wasdifficult to distinguish which phase was present.

'X-ray diffraction analysis on each sample indicated no preferredorientation of the ZrO, crystallites when a single fiber from each runwas examined.

' The above results and data of Table II show again the 40 thetemperature gradient peaked at 1,050F. Thefibers association of strengthand modulus with the presence of tetragonal ZrO and the retention oftransparency at elevated firing temperatures where such tetragonal 210is present. The moduli of elasticity and densities set forth in Table IIare all lower than that reported for zircon.

EXAMPLE 6 V A dispersion of colloidal silica in aqueouszirconiumdiaeetate was prepared by adding 900 g. of colloidal silica(Ludox AM) to 2,5 25 g. of aqueous zirconium diacetate. The resultingmixture (equivalent ZrOZESi O Z mole ratio of 111) was heated in a60-75C. water bath in a flask under water aspirator vacuum for about 14hrs., then concentrated for about 6 hrs. in a Rotovapor flask revolvingin a 40C. water bath under water aspirator vacuum, and then centrifugedfor 15-20 min. The resulting concentrate (about 1 liter,

. 60,000 cps. was pressurized at 50 psi from a bottle to a metering pumpand pumped at 200 psi through two drawn downwardly for 6 ft. in air(80F. 26% RH.) and passed upwardly around a' paper wick wet with2-ethyl-l-hexanol lubricant through an eyelet thread guide. Theresulting sized strand consistas fired were continuous and straight andwhen examined under a binocular microscope had a uniform diameter ofabout 15 p. and were round, smooth, waterclear, and transparent, andthey were much stronger than the green fibers. They were very flexible.

EXAMPLE 7 A viscous concentrate prepared like that of Example 6, havingan equivalent frog:sio mole ratio of l 1 and a viscosity of 46,000 cps.,was similarly spun into fibers, except that it was pressurized from abottle under psi to a pump and pumped at 230 psi to a spinnerette fittedwith one 400-mesh screen and having 30 4-mil round orifices. The spunfibers were lubricated with FC-43 fluorocarbon and drawn in the same waywith the rolls revolving at a rate sufficient to draw the fibers at 200ft./min., the aluminum belt moving into the 1,050F. furnace without thepreliminary heating with the heat lamps. The fired strands were wound onpaper cores using a tasland winder. The fibers in the strands afteremerging from the furnace were continuous. flexible, and straight,looking much like spun fiber glass, and under a binocular microscopeappeared waterclear, transparent, round, smooth, and glossy. The fibersas fired had a uniform diameter of about l5 u.

EXAMPLE 8 A viscous concentrate (140,000 cps., equivalent ZrO 2:Si O of1:1) was prepared like that of Example and fibers were s p u n ahd firedin a mai'iriaiiksiafilpie 7. They were spun by pumping the concentrateat 500 to 1,000 psi through a 22 a powder metallurgy stainless steelfilter and extruding through a spinnerette having 80 3-mil roundorifices and drawing the extruded fibers in air (78F, 24% RH) at therate of 250 ft./min.

The green strands were fired in air at 1,050F. for 6 min. like that inExample 6. A portion (26 g.) of the fired strands were randomly packedin a 4.2 in. ID. Vycor glass tube and fired in air at 950C. for 1 hr.,min. to consolidate or set the strands in the form of a mat or batt. Theresulting fired mat (4.2 in. dia. and 1.5 in. thick with a bulk densityof 0.08 g/cc.) was then tested to determine its sound suppressionproperty according to ASTM C-384-68. The mat was found to have a peaksound absorption coefficient of 0.95 at 1,600 cycles/see, showing thatthe refractory mat had excellent sound suppression properties at highfrequencies comparable to that of spun glass fiber. The fibers in thefired mat were very flexible and under a binocular microscope appearedwater-clear, transparent and round.

Another portion of the l,050F.-fired strands were refired in air at1,000C. for 1 hr., the individual fibers in the resulting refiredstrands being water-clear, transparent, and round under a binocularmicroscope. Chemical analysis of these refired strands revealed them tobe composed of 70.4 wt. ZrOZ and 30.1 wt. SiO2, which is equivalent to aZrO :SiO mole ratio EXAMPLE 9 A viscous concentrate (47,000 cps,equivalent ZrQgSiO; of 1:1) was prepared like that of Example 6 andfibers were spun ina marine r lik e Example 776; cept that they were notcoated with lubricant nor drawn in the form of a thread) by extrudingthe concentrate under a pressure of 190 psi through a spinnerette having80 3-mil round orifices and drawing the fibers in air (82F, percent RH),on a take-up drum at a speed of 250-300 ft./min. A portion of the greenfibers in the form of a coil was placed on a porous silica slab andfired in air from room temperature to 960C. and held at 960C. for 1hour. The fired fibers were flexible and strong, with a modulus ofelasticity of 15 X 10 psi and a density (determined by mercuryporosimetry) of 3.70 g./cc. Under a binocular microscope, they appearedwater-clear, transparent, round, and shiny. Another portion of the greenfibers were fired in air from room temperature to 1,100F. and held at1,100F. for 6 hr., and a portion of the resulting fibers were refired inair at 1,800F. for 1 hrs. and another portion refired in air at 2,200F.for 2 hrs. The 1,800F.-fired fibers, under a binocular microscope, werewater-clear, transparent, and round, and X-ray diffraction showed thepresence of only Zl'Og in the tetragonal form (relative intensity 100)and in the monoclinic form (relative intensity 5), the SiO; componentbeing undetected and apparently present in its amorphous form. The2,200F.-fired fibers, under a binocular microscope were opaque white,and they were fragile and sintered together at their intersecting pointsof contact. Under X-ray diffraction, they were shown to be composed of amixture of zircon (relative intensity 100). tetragonal Zr( (relativeintensity 7), and mono- The 9 60C.-fired fibers were evaluated asreinforcement for a plastic composite as follows. A methyl Cellosolvesolution of a thermosetting epoxy novolac resin was poured over aportion of the fibers to impregnate the same and the mixture heated inan oven to remove the Cellosolve solvent. The resulting tacky sheet wascut into an 0.6 X 6 in. strip and placed in a matched die mold at roomtemperature and hot pressed at 200-400 psi for 1 hr. at 350F. The moldedcomposite was then removed from the mold and post-cured in a 350F. ovenfor 4 hrs. The cured composite had a Barcol hardness of 76-80, and wasthen tested to ASTM D790-66, using a 30/1 span/depth ratio, and found tohave an unidirectional flexural strength at room temperature of 81.0 X10" psi and a modulus of 8.40 X 10 psi, showing that the fibers wereuseful as a plastic composite reinforcement.

EXAMPLE 10 A dispersion of colloidal silica in aqueous zirconiumdiacetate was made by mixing 800 g. of colloidal silica (Ludox LS30)with 2,244 g. of aqueous zirconium diacetate. This dispersion having anequivalent zirconia-to-silica mole ratio of 1:1 was concentrated underwater aspirator vacuum in a rotating flask in a water bath and thencentrifuged, the resulting concentrate having a viscosity of 740,000cps. and an equivalent solids content of about 49 wt. percent. Fiberswere spun from this concentrate from a spinnerette having 20 4-mil roundorifices using a spinnerette pressure of 550 psi., each of the extrudedfibers being drawn 4.5 ft. in air (F., 40% RH) by a take-up drum at arate of 400 ft./min.

A portion of the green spun fibers were subjected to thermal gravimetricanalysis (TGA) by heating them in air and measuring the weight lost asthe temperature was increased. in one run, the temperature was increasedat the rate of 10C./min., and in two other runs at the rate of 40C./min.In all three runs, the weight loss plotted against temperature increaseproduced a smooth curve, with the weight loss rate being greatest atabout 360-400C. Weight loss was essentially complete at about 560C. inall three runs, the total weight loss being 30-34 wt. percent based onthe weight of the initial or green samples, about 99 percent of thistotal weight loss occurring by the time the temperature reached 500C.Another portion of the green spun fibers were subjected to differentialthermal analysis (DTA) by heating the sample (powdered at about 100mesh) in air at a rate of l0C./min. As the sample was heated, a smallexotherm (probably due to dehydration) peaked at about 60-70 C. and wascomplete at about 98C. Another small exotherm occurred at 330C. and amajor exotherm (obviously removal of organic material) occurred sharplyat 382C, with a rapid fall off in this exotherm thereafter except for asmall exotherm occurring at about 419C. No further exotherm changes werenoted as the temperature was increased to 1,000C.

A portion of green fibers were fired in air from room temperature to1,000C. and cooled overnight, the fired fibers being continuous,straight, flexible and strong and under a binocular microscope appearedtransparent, clear, round, and about 14-20 p. in diameter. FIG. 3depicts the appearance of these fired fibers under a microscope. Aportion of the l,000C.-fired fibers were refired in air at l,200-I,300C.over a 2-hr. period, the resulting fibers being opaque and brittle, andunder a microscope having the appearance depicted by FIG. 4.

A hank of the l,000C.-fired fibers was dipped in a sizing solution of 4g. of L-I286 fluorocarbon in 200 g. of trichloroethylene and dried.Strands of the sized hank were coated with a sizing mixture of 1 partKraton lOl butadiene-styrene rubber, 1 part Nujol mineral oil, and 18parts toluol (these parts being parts by weight). The sized strands werethen woven by hand to form a l X 8 in. piece of fabric. The fabric wasplaced in a 950C. furnace and heated in air to l,000C. over about a30-min. period and held at that temperature for 30 min. The fired fabricwas cooled and found to be glossy, flexible, and strong, and under anoptical microscope the fibers in the fabric appeared water-clear andtransparent.

A piece of the fired fabric, which was about 2.5 inches in length, wastested to a load of 42 lbs. in tension on an Instron testing machinewith a load applied by movement of 0.05 inch per min.; though the testedfabric was apparently permanently stretched, it did not break.

EXAMPLE ll A viscous concentrate having a viscosity of 750,000 655.13"equivalent zrogfsiogmble ratio of I21 and 49 wt. percent equivalentsolids, was prepared like in Example IO. This concentrate was spun in alike manner. from a spinnerette having 3 -mil round orifices using aspinnerette pressure of 850 psi., the extruded fibers being drawn in air(80F, 50 percent RH) by a take-up drum at a rate of 430 ft./min. Twohanks of the fibers (measuring about 24 inches in length) were fired inair from room temperature to 980C. (over a period of 46 hrs.) and heldat this temperature for I hr. and then cooled overnight with the furnaceshut off. The fired hanks were sprayed with IMS silicon spray andbunches of sprayed hanks were coated withthe rubber-oil size used inExample IO. The sized fibers were then woven weaving.

EXAMPLE I2 A viscous concentrate having a viscosity of 180,000 cps.aided equivalent ZrO i SiO mole ratio of IzI was prepared like that ofExample 10 and similarly spun into fibers. These fibers were fired inair from room temperature to 500C. and refired at l,000C. for 1 hr. Thefired fibers were cut into 12 inch long hanks and bunches of fibers fromthese hanks were twisted into yarn and a 3 X 4 inch piece of fabricwoven therefrom. The borders of the fabric were coated with rubber-oilsize used in Example 10. The fabric was heated in air at l,000C. for ahr. The fired fabric was white, glossy,

and very flexible, the individual fibers, appearing trans parent under abinocular microscope.

' EXAMPLE I3 A number of different concentrates were prepared byconcentrating various dispersions of colloidal silica in aqueouszirconium diacetate and the concentrates were spun to form fibers, usinga spinnerette with six .3-mil round orifices, the spun fibers beingdrawn in air and wound on a reel. The composition of the concentratesand the spinning conditions are set forth in Table III. In all cases,the green (unfired) fibers were waterclear, transparent and round undera binocular microscope, and continuous and flexible.

The fibers were fired in air on a porous silica slab according tovarious schedules, I, II, etc., each schedule having from 2 to 4incremental steps, and the nature and appearance of the fired fibers atthe end of each schedule, and in some instances at the end of each stepin a particular schedule, were noted, the firing schedules and theresults obtained being set forth in Table IV. In Table IV, the nature ofthe predominant or major amount of fibers is given first, followed inparentheses by the nature of the minor amount of other fibers, if any.In all cases, the nature or appearance recorded was that observed undera power binocular microscope with oblique lighting. Except where anotation was made that the fired fibers were fragile, sintered at pointsof contact, or grainy, the fired fibers were flexible in nature andessentially continuous. Some of the fired fibers were examined by X-raydiffraction, and the results of such analyses are set forth in Table V.

TABLE III Composition of concentrate Takeup Equiv. Vis., Equiv. drum H7Comment ZrO cps. solids, Pres, speed, on Ru? SiO W X 10 wt. pl-I psift/min. spinning l 2:! 54 44.6 4 I25 85 spun well 2 l.5:l 56 47.5 3.5l20- -80 spun well I25 3" It] 27 ca45 3.5 I00 IlO spun well 4" lzl 4347.5 33-35 I25 50 spun fairly well 5"-" l:l 38 46 3-3.5 cal20 cal2O spunwell 6" l:l l6 3.5 125 I80 spun well 7 1:! .5 I04 50 3.5 I25 I25 spunwell 8 l:2 82 49.5 3.5-4 I25 I65 spun very well 9 1:3 calOO 51.5 35-4cal25 ca60 spun fairly well a Colloidal silica used in preparingconcentrate in these runs was Syton 200; all other runs used Ludox AM. hConcentrate for Run 4 was prepared from 240 ml. zirconium diacetate, 429ml. Syton 200, and 16.2 ml. acetate acid. c Concentrate of Run 4 spunfairly well but the green fibers were not very strong and they brokeinto long lengths after being wound. When this concentrate was dilutedwith water to l5.000 cps. and spun at psi, it spun hctter.

d Concentrate for Run 5 was prepared from I20 ml. zirconium diacetate.42 ml. S e,f Initial concentrate for Run 9 had too low a viscosity(78,000 cps) to spin well with the equipment used, so it wasconcentrated further (to about 100,000 cps.) and though it then spunfairly well. the green fibers were not strong and they broke intolengths Mu to 1 inch.

g Concentrate initially prepared from 240 ml. zirconium diacetate, 86ml. equipment used, so it was diluted for Run 3' with water to 27,000cps.

yton 200, and L6 ml. acetic acid, and resulting dispersion filtered.

Ludox AM, and 3.2 ml. acetic acid (HAc) was too viscous (240,000 cps.)to spin with the TABLE [V Equiv I ll III IV Run ZrO SiO, A. l40F., 2hrs.", B. refired IA at refired lA at A. RT to llF. B. refired MA atrefired MA at mole 340F., 2 hrs. then RT l800F., for 2 hrs. 2000F. for 2hrs. and held 6 hr. l800F. for 1 hr. 2000F. for 2 hrs. ratio to llOOF.and held 4 hrs.

l 211 opaque blk. (water translucent to opaque white, opaque blk. aboutequal opaque white, grainy clear and transparent) opaque white grainy(translucent amounts of opaque (clear, black), amber) blk & white grainygrainy 2 1.5:1 water clear & about equal opaque white, opaque blk. aboutequal opaque white, grainy transparent (opaque amounts of opaquetranslucent (amber, clear & amounts of opaque blk.) blk. 8!. white,transparent) blk. & white,

grainy grainy 3 l:l water clear & transparent & transparent milky waterclear & opaque white opaque white, grainy transparent slightly milky totranslucent transparent 4 lzl water clear & transparent & opatiuuhiteclear tan & opaque white opaque white, grainy transparent slightly milky(mil y transparent (milky transparent) transparent) l TABLE IV 2 lvwLwLM fi VI H Vll n V V l ll lX Run refired "M at A. RT to B. refiredVlA at RT to l|00F. and A. RT to 500C. B. refired VlllA RT to 500C. andheld I 1 l00F. and held I hr.

l800F., 1 hr.

and held I hr.

from 500C. to 950C 8!. held 1 hr.

hr. then l000C. l hr.

opaque white, fragile, sintered" opaque white, grainy, fragile,sintered' opaque white, grainy & fragile opaque white,

grainy & fragile,

sintered' opaque white, grainy 8L fragile, sintered opaque white, grainy& fragile, sintered water clear & transparent water clear 8L tramtparenlwater clear 54 transparent water clear & transparent water clear dttramtparent water clear (it transparent opaque blk.

water clear & transparent (opaque blk.)

water clear & transparent water elear lit transparent water clear, &transparent, to

milk'y to opaque blk.

water clear & transparent water clear & transparent water clear &ll'illlSP'tll'Cl'll paque lk p q white). grainy opaque blk. (opaquewhite) transparent, milky to translucent translucent, milky to opaquewhite transparent milky to translucent transparent 8: slightly milkywater clear & transparent water clear & transparent water clear 84transparent a-The fibers were also examined after the l4tl'1. and 340"firing ttteprt. but they exhibited no change in appearance from that ofthe green fibers, i.e. they were water clear and transparent. hRT" meanroom temperature (about 72F.) c-Thcse fired fibers had a modulus ofelasticity of 8 X if) psi. In a duplicate of Run 4, fibers were fired inair in a vitreous siliea tube from RT to l 100F. for 4 hrs. and refiredat llt00F. for 2.5 hrs. were predominantly water clear and transparent,with a minor amount being translucent to opaque black and white.

dln a duplicate of Run 6, fibers fired in a vitreous silica tube likethe duplicate noted in footnote c were water clear and transparent.e-See Table V for X-ray diffraction analyses of these fired fibers.f-Fihcrr; wprc fired in a duplicate Run 8 using the same Schedule IX,except that the l000C. step was held for L hrs., and were found to havea modulus of elasticity ofll X If) psi. g "Sintered" in Table IV meansaintered at point of contact,

TABLE V X'ray diffraction analysis Relative intensity of diffractionlines Run Firing Crystallite size A Cubic Tetragonal Monoelinic Othererystallites schedule 2 Hi8 ca 100 3 none 2 V l0 ZrSiO, (I00) 4 "I8 l0"100 5 none 4 V l0 2 7 ZrSiO, (I00) 8 VIB l0 lOO none 8 VII 10" 100eristobalite sio 7o 27 EXAMPLE 14 A dispersion of colloidal silica inaqueous zirconium (Ludox SM 15), which had been filtered through a 10 p.Millipore filter, and 227 g. of aqueous zirconium diacetate. Theresulting mixture was concentrated in a Rotovapor flask to about 50,000to 60,000 cps. The

viscous concentrate (equivalent ZrO :SiO of 2:1) was extruded underabout 70 to 90 psi through a spinnerette having six 3-mil round orificesand drawn in air by a takeup drum. A coil of the green fibers were firedin air from room temperature to 500C. and held at 500C. for 1 hr. Theresulting fired fibers were flexible, continuous, shiny, round, andpredominantly black, with a minor amount being water clear andtransparent, when viewed under a binocular microscope. When refired inair at 800C. for 2 hrs., more of the fibers were clear, the fibersotherwise being the same as the 500C.-fired fibers. when these 800C.-fired fibers were refired in air at 1,000C. for 1% hr., most of thefibers were water clear and transparent under a binocular microscope,with occasional black fibers being present, and in appearance wereotherwise the same as the 500C.- and 800C.-fired fibers. The1,000C.-fired fibers were found to have a density (as determined bymercury porosimetry) of 4.33 g/cc, an average modulus of elasticity ofabout 8 X 10 psi, and, when examined by X-ray diffraction, were found tohave crystallites that were predominantly tetragonal ZrO (relativeintensity 100) and to a minor extent monoclinic Zr (relative intensity3), the Si02 component not being detected and apparently in itsamorphous state; however,

EXAMPLE l5 Thirty g. of powdered polyvinylpyrrolidone were dissolved in130 g. water and the aqueous solution then stirred into 66 g. of. anaqueous colloidal silica (Ludox AM), the resulting solution having a pHof 7. Hydrated zirconium sulfate (31 wt. percent Zr02 equivalent) wasdissolved in the amount of 132 in 200 g. water, the resulting aqueouszirconium sulfate having a pH of about 1. The dispersion of silica inaqueous polyvinylpyrrolidone was stirred into the aqueous zirconiumsulfate, the resulting mixture having a pH of about 1. This mixture wasthen concentrated in a Rotovapor flask to obtain a viscous concentratehaving a viscosity of about 500,000 cps. This concentrate, containing anequivalent ZrO2:SiO mole ratio of 1:1 and an equivalent solids contentof about 27 wt. percent was then spun into fibers, using a spinnerettewith six 3-mil round orifices and operating at a pressure of 125 psi,the spun fibers being drawn in air and wound on a drum having a speed ofabout 100 ft./min. The spun fibers were heated in air from roomtemperature to 500C. and held at 500C. for 1 hr., and then refired inair at 1,000C. for 1 hr. The resulting fired fibers when viewed under abinocular microscope had a diameter of about 7 1 and were water clearand transparent, and were essentially n n que.flsxiblslaaqstr na,

EXAMPLE 16 aqueous zirconium diacetate, then adding 45.2 cc. of a 28saturated solution of ferric nitrate (containing 9.6 g. of equivalent FeO the resulting mixture being equivalent to 95 wt. percent of a mixtureof ZrO and SiO in a mole ratio of 1:1 and 5 wt. Fe O The resultingmixture was concentrated in a Rotovapor flask, halfsubmerged and rotatedin a 3040C. water bath. A portion of the resulting concentrate (about40,00050,000 cps.) was spun into continuous, straight fibers with aspinnerette having six 3-mil round orifices using a spinnerette pressureof about psi., the extruded fibers being heated with two infrared lamps,and drawn about 3 ft. in air by a takeup drum at 120 ft./min. Thesegreen spun fibers were flexible and under a binocular microscope, wereclear, transparent, light yellow in color, and round. Portions of thegreen spun fibers were fired in air at different temperatures and theappearance and nature of the fired fibers detennined. Results aresummarized in Table VI.

TableYl Strength, flexibility, and appearance Run Firing of fired fibersunder microscope 1 500C. for A hr. strong, flexible, light gold, clear,

I transparent 2 500C. for 1% hr., then to 600C.

strong, flexible, light gold, clear, transparent 3 500C. for V2 hr.,then strong, flexible, slightly darker gold,

to 900C. clear transparent 4 500C. for 1 hr., then to strong flexible,orange red,

1000C. and held 1 hr. transparent V 5 500C. for 1 hr., then to weak,brown-red, opaque 1 C. and held l hr.

EXAMPLE 17 speed of 300 ft./min. The spun fibers were straight,

flexible and essentially continuous in length. These fibers, under abinocular microscope were light yellowgold in color, clear, andtransparent. Hanks of about 2 ft. in length were fired in air from roomtemperature to 800C., and allowed to cool with the furnace overnight,the fired fibers being gold in color, shiny, flexible, and strong, theirlength having shrunk to about 17.5 in. Under a binocular microscope,they were gold in color, clear, transparent, and round with diameters ofabout 15 t. Some of these 800C.-fired fibers were refired in air at950C. for hr., and the refired fibers were orange-red in color, flexibleand stronger than those fired to 800C., and under a binocular microscopestill appeared clear and transparent.

Another portion of the 800C.-fired fibers were refired in hydrogen fromroom temperature to 800C., and cooled in hydrogen, to produce shinyblack fibers which were opaque but still strong and flexible, and whichwere attracted to a magnet but not electrically conductive.

Other portions of the above concentrate were not drawnbut rather wereextruded and allowed to fall from the spinnerette by their own weightand collect on a substrate in a random fashion. These extruded fibers(about 25-35 p. in diameter) were heated in air to 800C. Portions of the800C.-fired C.-fibers were refired at higher temperatures for I hr. andtheir relative strength and microscopic appearance and their crystalliteidentity under X-ray diffraction was determined, as noted in Table Vll.

Table Vll Strength, flexibility, and

X-ray diffraction Run Firing,C. appearance rel., intensity I 850 strong,flexible, gold, cubic ZrO I) clear, transparent 2 900 strong, flexible,gold, cubic ZrO (I00) clear, transparent 3 950 strong, flexible,slightly tetra. ZrO (100) darker gold, clear, mono ZrO, (I) transparent4 975 strong, flexible, slightly tetra. ZrO, (I00) darker gold than Run3,. mono ZrO (4) clear, transparent 5 I000 strong, flexible,orangctetra. ZrO (I00) red, clear, transparent mono ZrO (l0) 6 1025strong, flexible, copper tetra ZrO (I00) red, clear transparent to monoZrO, (l5) (some translucent) 7'" I050 weak, rust red, translucent tetraZrO (I00) to opaque mono ZrO, (60) ZrSiO. (70) (possibly some aFc,O;,) 8I075 weak, rust red, translutetra ZrO, (I00) cent to opaque, similar tothose of Run 7 mono ZrO, (50),

ZrSiO. (80) (possi' bly some aFc,O,)

Fired fibers of this run are depicted in FIG. I; the fibers of Run 3 hadessentially the same appearance as those in FIG. I, except for color.

EXAMPLE 18 About 1 g. of the black fibers of Example 17 were placed in aloose fashion in a ceramic crucible (1 inch in diameter and 1% inches inheight) and about g. of molten aluminum was poured on the fibers in thecrucible and the mixture allowed to cool to form a fi- 'ber-reinforcedaluminum composite. Upon examination under a binocular microscope, thefibers in the aluminum matrix appeared gold in color and to have beenwetted by the aluminum. The gold color was apparently caused byreoxidation of the reduced iron oxide when the latter was heated by themolten aluminum exposed to the air.

I 800C.-fired fibers of Example 14 was determined.

After 48 hrs. immersion in a concentrated hydrochloric acid bath at roomtemperature, the fibers were still gold in color, flexible and appearedunchanged, though the acid bath was yellowish. The acid treated fiberswere washed with water and immersed in fresh concen trated hydrochloricacid; after 48 hrs., the fibers were still gold in color, flexible, andapparently unchanged, and the acid bath was essentially colorless; after8 additional days of immersion, the fibers and acid bath appearedunchanged.

When another batch of the extruded 800C.-fired fibers was immersed in 28wt. percent aqueous sodium hydroxide, after one week the fibers stillappeared gold, but were brittle and fragile, though the caustic bath wascolorless.

EXAMPLE A viscous concentrate was made like that of Example to preventcontact of the fibers with one another due to currents of air (78F., 56%RH) in the room. The fibers were drawn by and wound on a drum having aperipheral speed of 700 ft./min., the drawn fibers having a diameter ofabout I5 s. The fibers were fired in air from room temperature to 800C.and held at 800C. for 1 hr. The fired fibers were strong and flexibleand under a binocular microscope, were gold in color, clear, transparentand round.

A 1.5 X l2 inches piece of fabric was woven from the fired fibers by aprocedure similar to that used in Example 10, using lMS Silicon SprayS5I2 as a size on the hanks to facilitate weaving. The fabric wasunifonnly and tightly woven. The fabric was placed in a 500C. furnaceand heated in air to 820C. over a 1.5 hr. period. The fired fabric wasgold in color, shiny, and very flexible, the diameter of the fibers inthe fired fabric being uniformly about 13-15 p. in diameter.

The fired fabric was placed in a tube furnace which was hotter in themiddle than at the ends, and the fabric was heated in air, thetemperature in the middle of the furnace being about 950C. or above. Theresulting fabric had a gradation in color from one end to the other,ranging from a metallic copper-red at one end (which was near the centerof the furnace) to a metallic gold at the other end. Portions of thefabric had become overheated in the furnace and were dull reddishbrown,brittle and fragile, the other portions being flexible and having thegradation described above.

EXAMPLE 21 A viscous concentrate having a viscosity of 580,000 cps.,equivalent 58.4 percent solids and IZrO :lSiO was prepared like that ofExample 20. The concentrate was spun into fibers from a spinnerettehaving 20 3-mil round orifices, using a spinnerette pressure of 775psi.- The spun fibers were drawn in air (75F., 30% RH) using a take-updrum having a peripheral speed of 300 ft./min. Some of the spun fiberswere fired in air from room temperature to 850C. over a hr. period andthen held at 850C. for 1 hr. The fired fibers were flexible, gold incolor, and under a binocular microscope appeared clear, transparent,smooth and round with diameters of 15-20 .4.. A 4 X 6 inch piece offabric was woven from some of these fibers and fired in air to 800C. toburn off organic size (IMS Silicon Spray and rubber-oil) used inweaving. The fired fabric was flexible and gold in color with a metallicsheen.

Other portions of the spun fibers were fired in air from roomtemperature to 900C. and held at 900C. for 1 hr. These fibers were thenrefired in hydrogen from room temperature to 770C. and held at 770C. for2 hrs. The resulting fibers were black in color, opaque, shiny,flexible, and were attracted by a permanent magnet. The fired blackfibers were sized with the same rubber-oil mixture used in Example 10and woven to form a 3 inch X 4.5 inch piece of black fabric. The sizingwas removed from the fabric by washing it in toluene. The black fabric,like the black fibers from which it was made, was shiny and flexible andwas attracted to a magnet. The application of an oxypropane torch to aportion of the black fabric produced a inch gold spot. When the end of ainch O.D. copper tube heated in a bunsen burner flame was applied to theblack fabric, a gold ring was formed. A platinumrhodium wire, heated toa bright red color and shaped in the form of the letter S, when appliedto the black fabric for about 0.5 min., branded a gold-colored S on thefabric. When similarly branded with an even hotter wire, a copper-goldcolored brand formed on the fabric.

EXAMPLE 22 A dispersion of colloidal silica in an aqueous Zirconiumdiacetate was made by mixing 5.88 g. of CoCl '6H 0, 200 g. of colloidalsilica (Ludox AM), and 560 g. of aqueous zirconium diacetate. Thedispersion was filtered through a Whatman No. 54 filter paper andconcentrated under vacuum, using a 3040C. water bath. The resultingviscous concentrate (lZrO :1SiO was spun using a spinnerette having six3-mil, round orifices and a spinnerette pressure of 120 psi. The fibersas spun were drawn and wound by a take-up drum having a speed of 400ft./min. These fibers were continuous, straight, flexible, and lavenderin color and, under a binocular microscope, appeared clear, transparent,and round. When fired in air from room temperature to 500C., the fibersremained flexible and were very light lavender in color and under abinocular microscope appeared clear, transparent, and round. Refiringthe fibers in air for A hr. at 800C. and then refiring them in air to950C. also produced fibers which were flexible, still light lavender incolor after both refirings and, under a binocular microscope, were clearand transparent, and round.

EXAMPLE 23 Continuous, straight, flexible fibers were producedfrom adispersion prepared and spun like that of Example 22, except that twiceas much CoCl 6H O was used (i.e. l 1.8 g.), and the drawn fibers werewound at a rate of 220 ft./min. As spun, the fibers were lavender-bluein color and, under a binocular microscope, clear, transparent andround. When fired in air from room temperature to 900C., the firedfibers were flexible and slightly darker than those of Example 22 withthe blue color more pronounced, and were still clear, transparent andround under a binocular microscope.

EXAMPLE 24 A dispersion of colloidal silica in aqueous zirconiumdiacetate was made by mixing 600g. of colloidal silica (Ludox LS) and1,683 g. of aqueous zirconium diacetate. After stirring the mixture forabout 15 min., it was vacuum concentrated for about 11 hrs., using a40C. water bath. The resulting concentrate (1ZrO :lSiO 70,000 cps.), wasspun from a spinnerette having 10 4-mil round orifices, using aspinnerette pressure of 250-300 psi. The fibers as spun were drawn inair (79F., 32% RH) and wound on a take-up drum having a speed of 300ft./min. Coils of these fibers were placed in an autoclave and heatedfor about 2 hrs. to 400C. under a purged helium atmosphere of about1,000 psi. The autoclave heat was turned off and pressure released,allowing the fibers to cool. The fired fibers were opaque and black incolor. Part of these black fibers were heated in air from roomtemperature to 500 C., and held at 500C. for about 2 hrs., the resultingfired fibers being fairly strong and water clear and transparent under abinocular microscope.

The black fibers autoclaved at 400C. were found to be very porous, witha surface area of 188 m /g., and the fibers refired to 500C. in air werealso found to be very porous, with a surface area of M6 m /g. (thesesurface area measurements being made by the continuous BET nitrogenadsorption method). Said refired fibers were also found to have adensity of 2.08 g/cc. (as determined by mercury porosimetry), indicatingthe fibers had a pore volume of about 40-50 percent. The modulus ofelasticity of these refired fibers was in the range of 3 to 4 X 10 psi,and they were very flexible.

Said fibers refired at 500C. were evaluated as adsorptive packingmaterial for a gas chromatographic column as follows. A portion of saidrefired fibers were packed in straight glass tubes (A inch O.D., 1% feetlong) and the packed tubes installed in a Hewlett- Packard gaschromatograph (Model 810), using a thermal conductivity detector. Thepacked columns were evaluated by injecting various samples of gaseousmixtures and liquid mixtures into the input ends of the columns,operating the columns with normal through-put of helium carrier gas atvarious temperatures from ambient to 250C., and observing the timerequired for each component of the sample mixtures to pass through thepacked columns and register on the detector. Operation at ambienttemperature resulted in selectivc retention and separation of C to Chydrocarbon gases of a natural gas sample, and various halocarboncompounds present in Freon l2 and Freon 1 l3 samples. Operation at 250C.resulted in retention and separation of various C to C hydrocarbon in asample of mixed n-alkanes, and retention but incomplete separation of Cto C alcohols in a sample of mixed aliphatic alcohols. Returning thecolumn to ambient temperature operation after conditioning it at 250C.for 40 hrs. gave the usual increase in retentivity. These results showthat the refractory fibers exhibit adsorptive properties similar toconventional adsorptive packings of alumina, silica gel, and charcoal.Similar useful results were obtained by using the same refractorycomposition in granular form as an absorptive material.

EXAMPLE 25 Unfired or green fibers of Example 21 were heated in anautoclave to 400C. and held at 400C. for 1 hr. under a purging heliumatmosphere at a pressure of 1,000 psi in the manner of Example 24. Theautoclaved fibers were fired in air from room temperature to 500C. andheld 2 hrs. at 500C. The resulting fired fibers were predominantly goldin color, flexible and under a binocular microscope, transparent with afew small areas of which were copper in color and translucent to opaque.These fibers were found to have a surface area of 138 m lg. (as measuredby the BET nitro gen adsorption method), showing them to be quiteporous.

EXAMPLE 26 A first batch of an aqueous dispersion of silica was preparedby adding 1.7 liters of colloidal silica (Ludox SM-30) to 2 liters ofaqueous zirconium diacetate, and stirring the mixture for about 1 hr. Asecond batch was prepared similarly by adding 7.1 liters of saidcolloidal silica to 8.1 liters of said aqueous zirconium diacetate.These two batches were then mixed together and vacuum concentrated in asteam-heated vessel to a viscosity of 7 X cps. The viscosity of thisconcentrate was lowered by the addition of water and reconcentrated to340,000 cps.

The above-prepared concentrate, having a viscosity of 340,000 cps and anequivalent ZrO zsiO mole ratio of 12.2, was extruded through five26-gauge hypodermic needles (parallel spaced on 0.5 inch centers) undera pressure of 400 psi, the fibers as extruded being blown by impingingstreams of air under pressure meeting at the outlet ends of the needles.The impinging streams of air were supplied from two /2 inch lD pipespaced 1% inch apart on center, each pipe having fifteen 0.040 diameterholes spaced inch apart. The pipes were set back from the outlet ends ofthe needles and positioned perpendicular thereto, one such pipe beingabove the needles and the other below the needles. The extruded fiberswere blown in this manner onto a carrier web and deposited thereon inthe form of a non-woven mat. A plurality of mats were made in thisfashion. When the blowing streams of air were applied at 10 psi., theresulting blown fibers were about 1 to 2 inches in length and had adiameter of about 0.001 inch. As the blowing air pressure increased, theblown extruded fibers had a diameter of 0.0005 to 0.00025 inch. The matswere from l/32 to V8 inch in thickness and looked like spun whitecotton. The green mats were fired in air from room temperature to 500C.and held at 500C. for 30 to 60 min. A portion of these fired mats wererefired in air at 1,000C. for l hr. The 500C.-fired mats were flexibleand stronger than the unfired or green mats. The 1,000C.-fired mats werestill flexible but more rigid than the 500C.-fired mats. The fibers ofthe 500C.-fired mats and the l,000C- fired mats, when viewed under amicroscope, were water clear and transparent and the continuous fibersEXAMPLE 27 Fibers were prepared in the same manner as Example 24, exceptthat the fibers were merely extruded from the spinnerette (that is, theywere not drawn by a takeup drum). The extruded fibers were allowed tofall 2 ft. by their own weight and collect in a random fashion on apaper substrate to form a mat. These fibers had diameters of about 0.002inch and were fairly wet with indipoints of the fibers in the matslooked like fused glass joints, but they were continuous rather thanshort fibers.

EXAMPLE 28 A dispersion of colloidal silica was prepared by dis solving1.78 g. of CuCl -2H O in water and adding it to 40 g. of colloidalsilica (Ludox AM), and Stirring the resulting mixture into 1 12 g. ofaqueous zirconium ace tate. The mixture was further mixed in a highshear mixer and vacuum concentrated in a rotating flask in 3040C. waterbath. The resulting viscous concentrate (lZrO :lSiO was clear dark blue.Fibers spun from this concentrate were fired in air from roomtemperature to 900C, the resulting fired fibers being flexible, lightgreen in color and, under a binocular microscope, clear, transparent,and round.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodiment setforth herein.

1 claim:

1. A solid, transparent, essentially zircon-free refractory fiber, roundin cross-section, comprising a mixture of microcrystalline zirconia inpredominantly its tetragonal form and amorphous silica, the mole ratioof zirconia to silica in said fiber being on the range of 1.5:1 to 1:2.

2. Refractory fiber according to claim 1, wherein the mole ratio ofzirconia to silica is about 1:1.

3. Refractory fiber according to claim 1, having a density of 1.5 to 4.3grams per cubic centimeter.

4. Refractory fiber according to claim 1 having a modulus of elasticityof less then 25 X 10 psi.

5. Refractory fiber according to claim 1 having a modulus of elasticityof 7 to 20 X 10' psi.

6. Refractory fiber according to claim 1, further comprising a metaloxide.

7. Refractory fiber according to claim 1, further comprising 0.5 to 25weight percent of a metal oxide, based on the total weight of saidzirconia and silica.

8. Refractory fiber according to claim 1, further comprising an amountof a metal oxide sufficient to impart a color thereto.

9. Refractory fiber according to claim 1 further characterized as beinground, straight, and continuous.

10. Textile comprising fiber of claim 1.

11. Woven fabric comprising fiber of claim 1.

l2. Woven fabric comprising fiber of claim 1 and capable of having itscolor or tone changed by the application of heat in an oxidizing orreducing atmosphere.

l3. Non-woven textile comprising fiber of claim 1.

14. A mat comprising fiber of claim 1. I

15. An essentially zircon-free, solid, transparent refractory fiberround in cross-sections, comprising a mixture of microcrystallinezirconia and amorphous silica, said zirconia being essentially the solezirconium compound in said refractory and predominantly present in itstetragonal form, the mole ratio of zirconia to silica in said refractorybeing about 1:1.

16. A product comprising a plurality of the fibers of claim l5.

17. A method comprising extruding in air in the form of fiber a liquidcomposition comprising an aqueous mixture of colloidal silica andzirconia compound capable of being calcined to zirconia, the mole ratioof equivalent zirconia to silica in said liquid composition being in therange of 1.511 to 1:2, said liquid having an equivalent solids contentof to 55 weight percent, heating the resulting gelled fiber to removewater, organic material, and carbon therefrom and form refractory fiber,and heating the latter at an elevated temperature in the range of 900 to1,150C. for a sufficient period of time to form solid, transparent.essentially zircon-free refractory fiber comprising a mixture ofmicrocrystalline zirconia in predominantly its tetragonal form andamorphous silica.

18. The method according to claim 17, wherein said elevated temperatureis in the range of 950 to l,050C.

19. The method according to claim 17, wherein the mole ratio of zirconiato silica in said liquid composition and in said refractory fiber is1:1.

20. The method according to claim 17, wherein said liquid compositionhas a viscosity of 45,000 to 500,000 cps.

21. A method for forming fibers made of a transparent, solid,essentially zircon-free refractory comprising a mixture ofmicrocrystalline zirconia in predominantly its tetragonal form andamorphous silica, the'mole ratio of zirconia to silica in saidrefractory being about 1:1, which method comprises extruding throughorifices a viscous liquid comprising a dispersion of colloidal silica inan aqueous solution of zirconium diacetate, drawing the resulting fibersin air, heating the drawn fibers in air to remove water, organicmaterial, and carbon therefrom and form refractory, and further heatingthe latter at 900 to l,l50C. for a sufficient time to form saidrefractory fibers.

2. Refractory fiber according to claim 1, wherein the mole ratio ofzirconia to silica is about 1:1.
 3. Refractory fiber according to claim1, having a density of 1.5 to 4.3 grams per cubic centimeter. 4.Refractory fiber according to claim 1 having a modulus of elasticity ofless than 25 X 106 psi.
 5. Refractory fiber according to claim 1 havinga modulus of elasticity of 7 to 20 X 106 psi.
 6. Refractory fiberaccording to claim 1, further comprising a metal oxide.
 7. Refractoryfiber according to claim 1, further comprising 0.5 to 25 weight percentof a metal oxide, based on the total weight of said zirconia and silica.8. Refractory fiber according to claim 1, further comprising an amountof a metal oxide sufficient to impart a color thereto.
 9. Refractoryfiber according to claim 1 further characterized as being round,straight, and continuous.
 10. Textile comprising fiber of claim
 1. 11.Woven fabric comprising fiber of claim
 1. 12. Woven fabric comprisingfiber of claim 1 and capable of having its color or tone changed by theapplication of heat in an oxidizing or reducing atmosphere. 13.Non-woven textile comprising fiber of claim
 1. 14. A mat comprisingfiber of claim
 1. 15. An essentially zircon-free, solid, transparentrefractory fiber round in cross-sections, comprising a mixture ofmicrocrystalline zirconia and amorphous silica, said zirconia beingessentially the sole zirconium compound in said refractory andpredominantly present in its tetragonal form, the mole ratio of zirconiato silica in said refractory being about 1:1.
 16. A product comprising aplurality of the fibers of claim
 15. 17. A method comprising extrudingin air in the form of fiber a liquid composition comprising an aqueOusmixture of colloidal silica and zirconium compound capable of beingcalcined to zirconia, the mole ratio of equivalent zirconia to silica insaid liquid composition being in the range of 1.5:1 to 1:2, said liquidhaving an equivalent solids content of 15 to 55 weight percent, heatingthe resulting gelled fiber to remove water, organic material, and carbontherefrom and form refractory fiber, and heating the latter at anelevated temperature in the range of 900* to 1,150*C. for a sufficientperiod of time to form solid, transparent, essentially zircon-freerefractory fiber comprising a mixture of microcrystalline zirconia inpredominantly its tetragonal form and amorphous silica.
 18. The methodaccording to claim 17, wherein said elevated temperature is in the rangeof 950* to 1,050*C.
 19. The method according to claim 17, wherein themole ratio of zirconia to silica in said liquid composition and in saidrefractory fiber is 1:1.
 20. The method according to claim 17, whereinsaid liquid composition has a viscosity of 45,000 to 500,000 cps.
 21. Amethod for forming fibers made of a transparent, solid, essentiallyzircon-free refractory comprising a mixture of microcrystalline zirconiain predominantly its tetragonal form and amorphous silica, the moleratio of zirconia to silica in said refractory being about 1:1, whichmethod comprises extruding through orifices a viscous liquid comprisinga dispersion of colloidal silica in an aqueous solution of zirconiumdiacetate, drawing the resulting fibers in air, heating the drawn fibersin air to remove water, organic material, and carbon therefrom and formrefractory, and further heating the latter at 900* to 1, 150*C. for asufficient time to form said refractory fibers.